Merge pull request #16 from guangmingyao/yao_dev

Yao dev
Changed a section number for one lecture
Changed the order of the labs.
Minor wording to a problem involving an integrating factor.
Changes in the wording for the introductory modeling problems.
Added a section on variation of parameters for the 2nd order ODEs
Added a first draft for a new activity on second order DEs
Added a second part to the new worksheet.
Added an image to demonstrate the connections within the course topics.
Rearranged the activity on modeling.
Updated the modeling activity to include a problem on energy in the system.
Minor formatting changes for the 2nd order modelign activity.
Merge pull request #40 from KellyBlack/black_dev

Merge additional activities and a reordering of the activities based on the new schedule.